Method of roasting fines



y 1936- H. J; CORDY ET AL 2,047,995

METHOD OF ROASTING FINES I Original Filed Feb. 19, 1950 2 Sheets-Sheet l INVENTORS I V/W/qm J7 Burg oyne Fatented July 21, 1936 METHOD OF ROASTING FINES Henry John Gordy, Sulphide, and William John Burgoyne, Belleville, Ontario, Canada, assignors to General Chemical Company, New York, N. Y., a corporation of New York Original application February 19, 1930, Serial No. 429,691. Divided and this application July 6, 1933, Serial No. 679,188. In Canada February This. application. is a division of our copending application Serial No. 429,691, filed February 19, 1930, which is a continuation in part of our U. S. Patent No. 1,758,188, granted May 13,1930.

Our invention relates to a method and apparatus for the roasting of finely divided sulfide ores, flotation concentrates and the like, and more particularly to apparatus for roasting finely divided pyrites ores or flotation concentrates,

to thoroughly desulfurize the same and to produce sulfur dioxide forum in the manufacture of sulfuric acid. 7

The present practice in pyrites fines roasting generallyinvolves the use of mechanically oper ated multiple hearth constructions, such, for example, as the well known MacDougall, Herreshoff, and Wedge burners, and while these burners provide a very effective roasting of the fines, their complicated construction and operation involve considerable initial and maintenance. expense. I

As distinguished from the bed roasting operation of these burners, it has been suggested to 'roast the fines while in gaseous suspension,.

wherein the fines are either injected into a roasting chamber in suspension in the oxidizing gas, or are simply showered downwardly into the roasting chamber wherein they encounter cross or counter currents of the suspending gas.

This suspension roasting is best' applicablev when the fines are in a very finely divided state, e. g. as flotation concentrates, and it presents the notable advantage over the mechanical multiple hearth operation of considerably lowering the cost of production, by reason of the elimination of the involved and expensive rabbling and other apparatus which distinguishes the construction and operation of the mechanical burners. In the practice of this suspension roasting, however, and

particularly as applied to roasting pyritic fines,

dimculties of some moment are presented which must be overcome before the obvious theoretical advantages of this type of operation are practically available.

The most serious and perplexing diificulty encountered in the suspension roasting of pyrites fines is the objectionable formation of accretions of scar on the walls of the roasting chamber during the roasting operation. These scar masses assume imposing proportions with disconcerting rapidity, and in a relatively short time, masses weighing several hundred pounds are commonly formed. Unless these masses are removed, obstruction of the roasting passage will result and the operation will be otherwise seriously hindered. The scar masses may be manually broken away,

that the pyrites being roasted will not be per- 1 Claims. "(01. 75-9) or barred down from the walls periodically, but the frequency with which this operation must be repeated to keep the scar formation within reasonable limits results in inordinate interruption of the operation and, in addition, the wear and 5 tear on the apparatus occasioned by. the heavily falling masses raises even more serious objection to this manner of removal. Moreover, the problem of incomplete desulfurization is not solved thereby for the considerable amounts of undesulfurized material which are confined within the accretions pass off in the cinder substantially un-' affected.

We have investigated the scarring problem noted and, while the exact reason for its occurrence is rather difficult to determine, it would seem to be more or less .ascribable to the very pronounced tendency which the iron pyrites'exhibits to sinter at a particular stage of its desulfurization, at which stage it readily adheres to 20 the confines of .the roasting chamber and builds up accretions thereon. Investigation of a typical iron-pyrites ore in this relation appeared to indicate that at a point in its transition from FeS: to F8203 which would roughly correspond to the 5 oxidation of one atom of the sulphur in the FeSz, the partially desulphurized material' sinters much more readily than at prior or" subsequent stages in the desulphurization. This may be explained either on the basis that FeS is the major form 30 40 mitted to contact with the walls of the reaction chamber while it is in the transitory stigky'condition noted.

We have experimented with various methods of roasting the pyrites fines in suspension, in an endeavor to find a means of overcoming the efiect of this transitory sintering condition, andhave found that by injecting a quantity of the oxidizing gas adjacent the walls of the roasting chamber in the upper part thereof, that scarring of the chamber walls is eliminated, very thorough desulphurization of the fines is obtained, and concentrations of sulfur dioxide are obtainable which are very suitable for use in the manufacture of sulphuric acid. This action appears to be due to the interposition of a supplemental por tion or blanket of the oxidizing gas between the walls of the roasting chamber and the main suspension of the burning pyrites when theiatter is in the state conducive to scar formation. c

The present invention relates more particularly to a modification of our improved method for roasting sulfide fines claimed in our U. S. Patent the ac- "feeding mechanism taken'along the line 8-8,

of feed mechanism;

Fig. 4 is a sectional elevation of a form Fig. 5 is a horizontal section on the line Hot Fig. G is a horizontal section onithe line 66 I of Fig. 4; c

Fig. 7 is a sectional elevation of a modified form 'of feed mechanism;

; fled form of feed mechanism,

Fig. 8 is asectional elevatiog of another modi- Fig. 9 is a vertical section of a modified form of feed conveyor mechanism.

. Referring to the drawings, and in particular to Fig. 1, reference numeral 4 indicates a shaft burner comprising a shell 5, constructed of a suitable refractory material such as fire brick or- -the like, which defines a roasting or reaction or similar material is provided about the sides of the shell 5 to suitably reinforce the same.-

chamber 6 of cylindrical shape throughout the major portion of its length. A casing 1 of steel Chamber 6 is provided; with a feed inlet 8 in-the center of the burner crown 9; a trough-like cinder --'pitll, constituting the bottom of the chamber,

- which extends diametrically thereacross and is open at the base throughout its length to provide 8 an elongated cinder outlet l2; and a lateral gas outlet l3 in the wallet the chamber just above the cinder pit l-l. Suitably constructed work holes I4, l5 and I6 covered by detachable external closures l1, l8 and I9 are also provided to afford access to the interior of the roasting chamber.

Terminating in the feed inlet 8 is a feed mechanism 26. for introducing the fines and oxidizing gas into the roasting chamber. This feed mechanism is constructed as follows: A charging hopper 2|, into which the finely divided material to be roasted is introduced in any suitable manner, is

mounted above the terminates in a tubular charging conduit 22,, which conduit discharges into a cross indicated generally at 23. This cross comprises a lateral arm 24 to which is bolted a cap 25 provided .with a suitable .bore for the accommodationof conduit 22; a downwardly extending arm 26 above which the discharging end of the conduit 22 terminates: a lateral arm 21 to which is bolted a cap 28 having a bearing 29; and an hpwardly extendingarm 8| having a cap 52 detachably secured thereto in any well-known manner whereby access maybe readilyhad to the interior r the crbss. Within the, conduit. 22 is a helicoid conveyor 33 whose shaft 34- is mounted for rotation in bearing29 in cap 28 and bearing 35 provided in a plate 36 closing one end of the conduit 22.

' A cylindrical coupling 31 having top and hottaken along the line ao aeoo tom flanges 38 and 39 respectively is secured at its top flange 38lto the flanged bottom 4| of the arm 26 .of cross 23,.and retained intermediate the joined flanges 3s and 4| is a baflie plate. which extends completely-across the passage provided by coupling 31. Baifle plate 42 is provided with a central orifice 43, and below and in alignment with said orifice is a distributing cone 44 supportedby across beam 45 which extends diametrically across and is bolted or otherwise secured to the coupling 31 in any well-known manner. A rod 46 depends from the cone 44,- extending through a bore provided therefor in the crossbeam 45, and this rodterminates at its lower end in a larger distributing cone41. I

Another cylindrical coupling 48 having top and bottom flanges 49 and 50 respectively is bolted to coupling 31 through the medium of the flanges 49 and 39, and gripped intermediate the joined flanges is the flanged top 5] of a funnel 52.

A cylindrical member 53', having top and bottom flanges 54 and 55 respectively, is bolted to coupling 48 through the medium of the flanges 54 and 5 6, and'has its bottom flanges 55 mounted upon and bolted to the top flange 56 of a sleeve 51 forming, with cylindrical member 53, a cylindrical casing which is set snugly into the feed inlet 8 and is'supported therein by means of a circum cylinder forming with a sleeve 69 and the vertical passage provided by the super-'jaoent couplings 48, 31 and the arm 26 of cross 23, a sectional ore inlet, conduit.

The cylindrical gas-distributing sleeve 60, having a top flange 6| is carried on the headed ends 62 of a number of suspension rods 63 extending through said flange, at least four of said rods spaced equi-distantly around the flange 6| being ordinarily desirable. These rods extend through suitable bores provided in flanges 59' and 50 and are retained on the lattes by means of adjustable and the clearance thereby presented will permit nular gas inlet chamber 61 is thereby provided 60 between the sleeve 60 and these surrounding members. The annular mouth or passage 68 of this latter inlet chamber 61 is defined by the rim of the flange or deflecting member 69 of sleeve 60 and the adjoining inner surface of the inlet sleeve 51, and by suitable vertical adjustment of the sleeve 60 by means of the suspension rods 63 the size of the mouth or passage 68 may be varied as desired, whereby a greater or less amount of F oxidizing gas may be admitted therethroughinto the roasting chamber from the inlet chamber 61.

As shown in Figs. 1 and 3, a series of suitably spaced apertures 1| for admission of the oxidizing gas is punched inwardly in the wall of the sleeve 60 at points therein below the bottom of cylinder 59, all of which apertures give access to the interior of the ore inlet conduit in .substantially the same general direction as regards the circumference of the sleeve 60. Spaced lower down in the wall of the sleeve is. another series of similar inwardly punched apertures 12 which give access to the interior of the sleeve in a direction substantially opposite to that of the upper series. projects from the casing 53, and to the flanged end 19 of said extension is bolted the flanged end 15 of a pipe 16 leading from a suitable fan blower a (not shown), whereby the oxidizing gas may be 15 blown into the feed mechanism.

Beneath. the cinder outlet I2 and extending in diametrical alignment therewith is a discharge conduit. 11 in which is mounted in any suitable fashion a helicoid conveyor 18 for'continuously for the operation noted is afiorded by the work I hole I l.

The gas outlet I3 leads to adust chamber 82 of any well-known type, part of which is shown in Fig. 1, wherein the gases are separated from the entrained dust prior to their utilization in the manufacture of sulphuric acid. In view of the fact that it is desirableto maintain a substan tially neutral, (i. e., atmospheric) or a slightly positive pressure above the cinder outlet I2 as noted in detail hereinafter, provision must be' made for drawing the dust-laden gases through the dust chambers, and this may be accomplished by maintaining a suitably reduced pressure at the outlet for the dust-free gases from the dust chambers (not shown), by any wellknown means such as a fan blower.

The modified form of feed mechanism, 85, 86

and 81, of Figs. 4 .to 8 inclusive, are in manyrespzcts similar to feed mechanism 20. Structural elements of the modified forms whichare the same or substantially the same as the corresponding parts already described in connection with Fig. 1 are indicated by like reference characters.

Referring to Fig. 4, a cylindrical coupling 88, flanged on both. ends, is bolted or otherwise attached to the upper flange of the coupling 31. The lower flange of the semi-cone shaped member 89 is interposed and clamped between the adjacent flanges of couplings 31 and and thus supported in the position shown. Attached to the upper end of the coupling 88 is a cap 90 through which passes an ore inlet pipe 9| having edge of the flange and the outer surface of the lower end of the inlet pipe 9I. The primary gas feed pipe 94 is suitably connected to the coupling 88 in such'manner that the axis of pipe 94 intersects the vertical axis of the coupling 88 and the subjacent ore inlet conduit formed by couplings 31, 58, terminal cylinder 59 and the distributing A tangential extension 13 (Fig.2)

sleeve 95. A chute, suitably supported by means not shown for feeding ore into the hopper 92 is indicated at 96. In some instances itmay be desired to replace I the chute 96 with a screw conveyor such as shown in Figs. 1 or 9. If such a change is desired, the lower flange II of cross 23, Fig. 1, may be placed directly over the cap 90, Fig. 4, and bolted to the upper flange of the coupling 88. In this instance, the end of the conduit 22 is positioned over the hopper 92, and the ore is thus fed into the inlet pipe 9|.

The gas distributing sleeve 95 is supported and may be adjusted vertically by the rods 63 in the manner already described in connection with Fig. 1. The sleeve 95 is of such diameter as to I have a sliding fitover the lower' end of the terminal cylinder 59, which construction permits a more rigid and accurate. regulation of the flange 69. r '20 As shown in Figs. 4 and 5, the walls of the sleeve 95 are punched inwardly to provide suitably spaced apertures at points below the end of the cylinder 59 and near the lower end of the sleeve 95. The openings 91 afford air inlets to 25 the interior of the sleeve 95, and the inwardlyb ent portions 98' serve to give the entering air a whirling motion within the sleeve. A second series of apertures 99 is spaced upwardly from the apertures 91. ,The inwardly-bent portions of the wall material are directed oppositely to those of the lower series and thus efiect a whirling of the incoming air or other oxidizing gas in the opposite direction.

The gasinlet chamber 61 is in communication '85 with the secondary gas inlet pipe I00 which is connected to the cylindrical member 53 in such manner that the axis of pipe I00 intersectsthe vertical axis of the feed mechanism as a whole. This radial connection between pipe I00 and the 40 cylindrical member 53 is shown in horizontal section in Fig. 6.

The feed mechanism illustrated in Fig. '1 differs from that described in connection with Fig. 4 in that the distributing sleeve IOI is provided with a single series of circular openings I02 near the lower end of the sleeve. In this instance the gas admitted from the inlet chamber 61 is introduced radially into the ore conduit.

In the modifications shown in Figs. 1, 4 and 7 the outside diametrof flanges 69 is slightly less. than the inner diameter of sleeve 51. This construction permits the insertion and withdrawal of the gas distributing sleeve without removal of the sleeve 51 from the crown of the burner. This construction also facilitates an adjustment of the annular' mouth 68 so that the form of the air blanket issuing therefrom will more nearly approach that of a cylinder, so that the air blanket first impinges on the veritcal walls of the burner some distance below the top.

In the modification of Fig. 8, the construction is essentially the same as in the form illustrated in Fig. 7. However, the gas distributing sleeve I03 is provided at its lower end with a plurality 'ofseries of circular openings I04,.any number of which may be put in communication with the gas inlet chamber 61 by proper adjustment ofthe cylindrical sleeve I 05 slidably mounted on the outside of the sleeve I03. The sleeve I0570 may be adjusted vertically by means of the racks I 06 fixed to the sleeve I05 and the co-operating pinions I01 keyed to the cross shafts I08. The

, shafts I08 pass through the wall of the sleeve 51 and are journalled in bearings suitably atrately. shaped.

pinions I01 may be operated by means of sep,- arate handles keyed to the ends of shafts I 08,

or'the rotation of the pinions It" in opposite directions at the same rate may be effected by a handle attached tothe end of one shaft, and pinions mounted on both shafts meshing with a pair of interposed idlers, the pinions and'idlers obviously being outside ,the s1eeves5'i. The adjustable sleeve I05 thuspermits accurate reg-,-

ulation of the quantityof air entering the ore inlet conduit from the gas inlet chamber 61.

The flange {09 is of a diameter greater than the interior diameter ofthe sleeve 57. This construction facilitates7adjustment of the sleeve I03 so that air entering the roasting chamber from the annular mouth may be directed along the roof of the burner crown.

A further advantage of this. construction is that the .air blanket may bemore accurately adjusted, and the quantity of air in the blanket reduced to a minimum. This makes available more air for: mixture with the ore in the main inlet conduit giving-a more. uniform mixture of suspended-ore in the air and a resulting more rapid and complete combustion in the furnace.

The gas distributing sleeve may be made ofany suitable material'such as wrought iron or cast iron. The latter ispreferred, however, as

it may be advantageously machined and accu- In Fig. 9 is fllustrated a modified form of. ore feed-mechanism which has proved highly satisfactory for furnaces for suspension roasting. In this modification, a part of the conveyor spiral immediately adjacent the discharge end of the conduit 22 is replaced by the two blade propellers Hi and I i2 keyed to shaft 34. The propellers are formed from disks radially slotted, with the adjacent sectors bent to effect the proper pitch. Propeller lli may be provided with say six blades, and propeller H2 with some greater number of blades, say twelve for example. As the spiral 33' pushes the ore forward in intermittent fashion, propeller HI divides each of'the small piles of finely divided ore delivered by the spiral 33' into six portions, and passes them on to the twelve bladed propeller 2 which picks up the orethus delivered and advances it into the main conduit of the feed nozzle in an al- This arrangement most continuous manner. eliminates the pulsating feed of ore which is effected by the continuous spiral 33 of Fig. 1, and

thus eifects more regular combustion and desulfurization in the furnace proper.

The operation of the invention is substantially as follows: ,The furnace first strongly preheated, for example. by means of one or more oil burners inserted through the work hole l4,

until a temperature is'obtained in the chamber '6 substantially above the ignition point of the iron'pyrites, for example about 850C. During the preheating operation work holes -l5 and I 6 are uncovered to provide flues forthe escape of the products of combustion. 1 I

Finely divided iron pyrites, which has been dried until the moisture is not-greater than 0.3% and preferably. between .1% and 2%, and of a particle size that 100% 'will pass,a -60 mesh (to .Zl, in any suitable manner.

the inch) screen,'is then .fed into the hopper From the hopper the fines are substantially continuously supplied to the helicoid conveyor 33 within the charge conduit 22, by means of which they are delivered into the passage in the arm it of cross .23.

-2,'o47,ecc

.Some' of the pyrites fines will pass directly 'in' any suitable and well-known manner.

.on the walls of the roasting chamber.

ing over the cone 44, the fines will be directed ontothe sides of the funnel 52, and thence will proceed into the terminal cylinder 59, at the lower portion or which they'will be spread out bythe distributing cone 4'! into a somewhat cylindrical-shaped sheet, in which form they will reach the gas distributing sleeve 60.

Air or oxygen may be used as the oxidizing gas, preferably the former, and either at atmospheric temperature, or preheated to any desired degree thereabove' which is below the ignition point of the pyrites, is blownthrough the pipe 16 and tangentially intothe' space between the cylindrical member 53 and the cylinder 59 (Figs. 1 and 2). The amount ofair introduced will depend upon theamount of sulfur in the fines to be desulphurized, the concentrationof SO: desired in the exit gases, and other factors evident to one skilled in the art, and the regulation of the air supply may be accomplished. 4

practice, determination of the S02 content of the exit gases and the character of the cinder will usually indicate the necessary regulation of the air supply to provide the desired results, the fines being supplied at a substantially regular rate. The pressure under which the air is introduced should be so regulated that a positive pressure very closely approaching atmospheric is obtained over the cinder outlet, a slight plus pressure of about .02" to .03" being suitable under most circumstances. I I

Upon introduction into the. feed mechanism the air will whirl around the cylinder 59, and

traveling downwardly, a major portion of the same will pass into the interior ofv the air distributing sleeve6ll through the passage 66 and the apertures II and 12 leading off inlet chamber 6'1, and the remainder will sweep directly into the roasting chamber 6 through the'mouth-68 of inlet chamber 61. The distributing sleeve 60 should be so adjusted by means of its suspension rods 63 that the size of the mouth-or annular passage 68 is such that only sumcient air is admitted therethrough to prevent scar formation This can be determined by varying the size of the mouth and observing the efiect upon the chamber walls through one of the work holes [5 or It. The amount of scar inhibiting air necessary is usually a minor proportion of the total amount of air admitted, and ranges generally from 5%- sleeve and the substantial-suspension of the intimately mixed air and fines obtained thereby, I

will pass on into the hot roasti'ng chamber 6. Upon entering the preheated chamber the suspension of fines and air'will be ignited and oxidation will thereafter proceedat a very rapid rate.

The oxidation of the iron pyrites is a strongly ture above the ignition point of. the pyrites, no

auxiliary heat is required. In operation the temperature will be quite high throughout the chamber 6; in the production of an approximately 10% S02 gas, from a typical iron pyrites ore, for example, the temperature in the roasting chamber was in the neighborhood of 1000 C.

Upon the entry of the turbulent gaseous suspension into the enlarged space presented by the roasting chamber, expansion of the suspension takes place, and this expansion is enhanced by the large amount of heat supplied by the combustion of the fines in addition to that emanating from 'the heated walls of the roasting chamber. This expansion tends to cause an appreciable proportion of the suspended particles to be thrown toward the chamber walls, and suchparticles as are in the transitory sticky condition hereinbefore discussed would under ordinary circumstances tend to adhere to the walls upon contacting therewith and rapidly build up accretions thereon.

The portion of air, however, which enters the roasting chamber through the annular mouth 98, simultaneously with the introduction of the main body of the suspension from the sleeve 60 serves to inhibit throughout the operation the usually inescapable scar formation.

In the case 'of the feed mechanism shown in Figs. 4 to 8, the finely divided ore is fed into the hopper 92 in a constant stream from the chute 95. Air or other oxidizing gas is introduced into theinterior of the coupling 88 through the primary inlet pipe 94, under a pressure sumcient. on passing through annular orifice 93 to induce an injector action which serves to draw in, and assist with the dispersion of, the ore in the vertical ore inlet conduit and also prevents the escape of air or gases through the feed pipe 9|. quantity of primary air introduced through pipe 94 varies from about 5% to 15% of the total amount of air introduced into the furnace. About to 95% of the total volume of airnecessary for combustion in the roasting cham ber enters the cylindrical member 53 and the inlet chamber 61 through the secondary inlet pipe I00. Radial introduction of air into chamber 61 is preferable because a better mixture of ore and air and more complete combustion are obtained. The major portion of the air in chamber 61 will pass intothe interior of the sleeves 95, ll and I03 through the slots 91 and 99 of Fig. 4, and the circular openings I 02 and HM of Figs. 7 and 8. In the form shownin Fig. 4, the mixing action is in some degree similar to that which takes place in the apparatus of Fig. 1,

with the probable difierence that the turbulent mixing action is a little more violent, thus tending to break up any individual spiral streams of finely divided'ore which may form. The. difference may be on account of the fact that substantially all the air entering the sleeve 95 from chamber 61 isadmitt'ed through the slots 91 and 99, as the passage between the lower end of sleeve 59 and the upper end of sleeve 95 is practically stopped off because of the" sliding fit between mixing of the finely divided ore and air is in some-degree differently affected, and in some cir- The ' practice can be eliminated.

cumstances for reasons not altogether apparent,

.better combustion in the roasting chamber and and it seems possible that as the proportion of 15 the F8203 increases, the tendency toward scarring decreases. On that basis it would seem that the provision of the air blanket between the walls of the chamber and the burning suspension need only be confined to the upper part of the cham- 20 her in order to prevent scar formation, and it may be that suchis the case in the operation of the present method as noted, although the particular path to the air blanket is more difiicult to trace the greater its penetration into the furnace. 25 On the other hand it is quite possible that the gaseous suspension of fines could be introduced under such pressure, for example, that the scar forming stage would occur at a point considerably lower down in the roasting chamber, in 30 which event it would be necessary to run the air blanket substantially throughout the length of the chamber.

In view of these considerations it is essential primarily that the air blanket be interposed between the walls of the chamber and the burning pyrites while the latter is in a state conducive to scar formation, regardless of its relative location in the roasting chamber while in that state. That such is the case in the operation of our method is evidenced by the fact that by suitably varying vertically the position of the flanges 69 and I09, and thereby the volume and direction of the air blanket admitted through the mouth 68 and the form and extent of the cone of dispersion of the air blanket, the formation of scar under such variations of the roasting operation as are involved in ordinary industrial Whether the scar inhibiting action of the air 50 blanket is due to an oxidizing eiiect, whereby the particles in the sticky stage are oxidized past that stage prior to contacting with the chamber walls, or to a physical efi'ect involving entrainment oi the particles, whereby they are prevented from contacting with the walls, or to a combination of both effects, is practically indeterminate and not particularly material. Regardless of the exact explanation it is certain that this auxiliary portion of the oxidizing gas is interposed in the upper portion of the furnace between the walls of the chamber and the burning fines while they are in a state conducive to the formation of scar onthe walls.

The length of the roasting chamber and the speed of introduction and travel of the fines and air should be such that a thorough desulphurlzation is obtained in the passage through the roasting chamber, as is obvious to one skilled in the 70 art.

By conducting the operation in accordance with the foregoing principles, the fines in suspension are quite thoroughly desulphurized by the time the cinder and gas outlets are reached 75 6 and by regulation of the pressure as hereinbefore noted, the major portion of the desulphurized particles, mainly in. the'form of iron oxide'collects in the cinder pit ll whence it is conducted away by the helicoid conveyor 18. The gases containing the SO passing into the dust-c'hanibers 82 carry ent ained therein an appreciableportion of the fihest particles, 1; 'e., dust, which dust, consisting mainly of iron: oxide is colhected in the dust "chambers, and thedust-free gases containing the S02 are drawn from the chamber and conducted away'for utilization in the production of sulphuric acid.

The roasting of pyritic fines in suspension ae- 16 cording to my inventionprovides an operation which, in practice, presents numerous advane tages over the various methods of burning such fines heretofore suggested. Its most important accomplishment is the re moval of the obstacle to efflcient practical op-- eration presented by the formation of the objec- .i tionable scar. The sulfide fines are, moreover, very thoroughly desulphurized, a very desirablefeature from-an efliciency standpoint: this very thorough desulphurization being also asoribable in the main to the elimination of the scar formation which serves to prevent thorough desulphurization of the pyrites fines.

In addition, the operation-requires but a slight amount of attention, and the initial .and main-i tenance expense involved are such as to permit production at a very low cost.

' While we have specifically noted the application of our invention to the roasting of, finely'divided iron pyrites ore or flotation concentrate, it is obvious that our invention is not confined to the use of that particular material but is applicable generally to other sulfide ores where the unclesirable formation of similar scar is involved. Copper pyrites ore and flotation concentratesot the same are characterized by a similar action,

and the same is also true of pyrrhotite although oxidizing gas constituting a major portion of the total volume of oxidizing gas required to support combustion, to form a more complete suspension of the ore in the oxidizing gas, and burning said chamber andsaid burning ore while the I latter is in a state conducive to scar formation 50 on said walls, whereby said scar formation is avoided.

2, The method or roasting a finely divided euifide ore which comprises forming a mixture or saidv ore in a primary volume of oxidizing, gas 65 constituting a minor portion of the total volume of oxidizing gas required to support combustion, 1

adding to the mixture; a secondary volume of total volume of oxidizing-gas required to support 70 combustion, to form a more complete suspension of the ore in the oxidizing gas, and burning said suspended ore in a roastinglchamber while maintaining asupplemental layer of the said.

oxidizing gas between the walls of said chamber and said burning ore while the latter is in a combustion, to form a more complete suspension or theore in the oxidizing gas, burning said susf of-oxidizing gas required to support combustion, adding to the mixture a secondary volume of said suspended ore in a roasting chamber w'hile ,maintaining a layer of gas between the walls of I .a more complete suspension of the ore in the air, passing said ore into a roasting" chamber. at a state conducive to scar formation on said walls,- whereby said scar formation is avoided.

3. The method of roasting a finely divided sulfide ore which comprises forming a mixture of said ore in a primary volume of oxidizing gas -5 constituting a minor portion of the total volume of oxidizing gas required to support combustion, adding to the mixture a secondary volume'of oxidizing gas constituting a major portion of the total volumerofoxidizing gas required to support 10 combustion, to form a more complete suspension ot-the ore in the oxidizing gas, and burning said suspended ore in a roasting chamber while i,

maintaining a supplemental layer of the said sec-- ondary oxidizing gas between the walls of said 15 chamber and said burning ore while the latter isfin a state conducive to scar formation on said walls, whereby said scar formation is avoided.

1. The method of roasting a finelydivided sulfide ore which comprises forming a mixture of .said do in a primary volume of oxidizing gas constituting a minor portion of the total volume of oxidizing; gas required to support combustion, adding to the mixture a secondary volume of oxidizing gas constituting a major portion of the 25 total volume of oxidizing gas required to support combustion, to form a more complete suspension of the ore in the oxidizing gas and burning said 1 suspended ore in a chamber, maintaining an en-- 30 velope of gas between the walls of said chamber and said burning ore -to appoint in'the chamber where suflicient oxidation of said ore has taken place toprevent subsequent formation of scar on. said walls.

i 5; The method of roasting a finely divided sul-, 5

fide ore which comprises forming a mixture of,

said ore in a-primary volume 'ofoxidizing gas constituting a minor portion of the total volume of oxidizing gas required to support combustion,

adding to the mixture a secondary .volume of oxidizing as constituting a major portion of the total volume of oxidizing gas required-to supportv pended ore in a chamber, and maintaining a portion oi. said secondary gas between the walls of said chamber'and said burningore to a point in said chamber where sufiicient oxidation of said ore has taken place to inhibit the subsequent tor- Y mation of scar on said walls.

a minor portionof the total volume of oxidizing gas required to support combustion; adding to the mixture a secondary volume or air constituting amajor portion of the total volume of oxidiz ing gas required to support combustion, to form temperature at which self-sustained combustion of the ore will be initiated and maintained, main-' taining a blanket oi. secondary :air between the walls ot said chamber and said burning-ore while p the ore is in, a state conducive to scar forma oxidizing gas constituting a major portion of the tion onsaid'walls, retaining said-suspension in the separating the desuli'urized ore from the gases.

'1. The method or roasting finely divided s i fide ore which comprises forming a mixture 01 I the ore in a primary volume of oxi in gas constituting a minor portion oi'the total volume of oxidizing gas required to support combustion, in troducing a secondary volume of oxidizing gas constituting a major portion of the total volume of oxidizing gas required. to support combustion,

radially into the mixture whereby a suspension of finely divided ore in the gas is formed, and burn- 1 ing said suspended ore in a roasting chamber while maintaining a lairer of gas between the walls of said chamber and said burning ore while the latter is in a stateconducive to scar formation on said walls, whereby said scar formation is avoided.

HENRY JOHN CORDY. WILLIAM JOHN BURGOYNE. 

